Crosslinking prepolymers of aldehyde condensate resins are made by two different processes, one of which produces products called resoles, and the other of which produces products which are called novolaks. In the process for producing resoles, sufficient trihydrogen donating material is mixed with the aldehyde to be capable of completing the crosslinking of the condensates. The reaction is terminated, however, before the material reaches the B-stage, so that it is still water soluble and consequently viscous and sticky. When attempts are made to dry such resoles, particularly with heat, the reaction usually goes beyond the B-stage to produce some crosslinking, and therefore this process is not used commercially for molding compounds.
In the case of novolaks, the reaction between the aldehyde and the hydrogen donor, as for example phenol, is carried out with a deficiency of the aldehyde i.e. no more than a 1 to 1 mol ratio so that the reaction will be stopped without proceeding beyond the B-stage. These materials usually are dried and are ground into powders which are then mixed with further hydrogen accepting material, as for example hexamethylenetetramine or paraformaldehyde to produce a powderous mixture which can be fused and molded into a desired shape. During molding, crosslinking takes place to produce a thermoset or C-staged material. Novolak materials usually cost at least 11/2 times that of resoles, but they have the advantage that they can be handled as powders and have a long shelf life.
We are aware that novolak resins have been used in conjunction with organic fillers, as for example saw dust; and further have been used with bulky light weight inorganic fillers, as for example asbestos, to produce molded articles, as for example brake lining. In the case of the reaction of the novolak with hexamethylenetetramine, ammonia is given off, and in all of the prior art processes including those mentioned above, the molded articles produced have been sufficiently porous to vent the ammonia that is generated without disrupting the molded shape. To our knowledge, no one has satisfactorily produced molded articles using novolaks filled with inorganic fillers, because of the problems that are created by the ammonia produced during molding.
An object of the present invention is the provision of a new and improved molding compound comprising a high density inorganic filler and a novolak which can be molded without blisters and cracking.
Another object of the present invention is the provision of a new and improved sheet molding compound and process of making same which utilizes novolak resins.
A further object of the present invention is the provision of a new and improved process wherein novolak resins are produced from monomer while dispersed throughout high density inorganic fillers, following which a hydrogen donor and particularly hexamethylenetetramine is blended therewith and formed into sheet molding compound.
Further objects and advantages of the present invention will become apparent to those skilled in the art to which the invention relates from the following description of the preferred embodiments.